Stand the watermelon half cut side down on a cutting board and slice away the rind, leaving a solid block of melon.,______,Pour the vinegar into a small saucepan and bring to a simmer over medium-high heat.,Cook until reduced to a thick syrup consistency.,Set aside.,Heat a nonstick grill pan over medium-high heat.,Drizzle just enough olive oil over watermelon slices to thinly coat and place on hot grill pan.,Grill each side about 2 minutes until grill marks appear; transfer to a plate and season with salt.,To assemble the salads, put about 1/4 cup of baby arugula on a serving plate, followed by a grilled slice of watermelon in the center, and top with a tablespoon of crumbled cheese and another 1/4 cup arugula.,Add another watermelon slice and another tablespoon of cheese.,Repeat with remaining ingredients.,Finish each salad with a very light drizzle of olive oil and balsamic syrup.,Dust with black pepper and serve immediately.